In the past, various different techniques are believed to have been utilized in an attempt to flow air through a contained space of a system including an apparatus for conditioning the temperature of the air with the rate of such air flow being related to the static pressure associated with such system. Both the speed and torque of an electric motor driving a fan or blower to effect air flow through the system are affected by the static pressure in the system. The rate of air flow (CFM--cubic feet per minute) through the apparatus also affects the motor speed and torque.
One approach of the past involved the rather laborious and time consuming matching of motor speed and torque with the proper fan to approximate the desired air flow rate for the particular contained space and static pressure of the particular apparatus or system in which such apparatus was employed However, this did not accommodate variations in the static pressure in the system caused by alterations in the system such as opening, closing or adjusting of a vent or the like connecting a conditioned space in air flow relation with the system.
If the fan or blower utilized in the past was of the fan or blade type, a decrease in the static pressure acting on such fan resulted in an increase in the speed of the fan and the electric motor driving it. Conversely, if the static pressure acting on the fan was increased, the speed of the fan and electric motor decreased. Thus, the speed of the fans and electric motors utilized in the past varied inverselY with a variation of the static pressure in the system.
As recognized in coassigned U.S. Pat. No. 4,806,833, incorporated herein by reference in its entirety, a decrease in the static pressure acting on a squirrel cage blower results in a decrease in the speed of the squirrel cage blower and the electric motor driving it. Conversely, if the static pressure acting on the squirrel cage blower is increased, the speed of the squirrel cage blower and its driving electric motor is increased. Thus, it was found that the speed of the squirrel cage blower and its driving electric motor varies directly with a variation in the static pressure. Accordingly, U.S. patent No. 4,806,833 discloses a method of operating a system for conditioning air including a variable speed blower for flowing the conditioned air through a contained space having a static pressure therein. In this system, the speed of the blower is set to affect a preselected flow rate at an existing static Pressure in the contained space and the speed of the blower is altered only in response to a variation in the static pressure and only in following relation with the static pressure variation. The speed alteration of the blower is sensed, and the speed of the blower is altered in following relation with the sensed speed alteration to establish the preselected flow rate through the contained space at the varied static pressure acting on the blower.
Although this approach is an improvement over the prior art, it is only a rough approximation of the characteristics needed to achieve constant air flow with respect to changes in static pressure. In general, the systems of the prior art worked reasonably well over a limited range of air flows and static pressures by employing a single slope speed compensation technique. It has now been recognized that a much more accurate approach to providing speed compensated torque which can be implemented in an air handling system is required. Such speed compensated torque would then allow the blower motor to maintain air flow in the system independent of variations in the static pressure in the duct work.